Stimulation of the mesencephalic ventral tegmental area blunts the sensitivity of cardiac baroreflex in decerebrate cats.

We have examined for the first time whether electrical stimulation of the mesencephalic ventral tegmental area (VTA) or the substantia nigra (SN) was capable of suppressing cardiac baroreflex sensitivity in decerebrate cats. After decerebration was performed by electrocoagulation at the precollicular-premammillary level and inhalation anesthesia was stopped, the animals were able to show spontaneous motor activity intermittently. Electrical stimulations of the mesencephalic areas (the VTA and SN) for 30s were conducted with a monopolar tungsten microelectrode (current intensity of pulse trains, 50-100 μA; frequency, 40-50 Hz; pulse duration, 0.5-1.0 ms), without producing tibial motor discharge. Stimulation of the VTA evoked the significant increases in heart rate (HR, 12 ± 2 beats/min) and mean arterial blood pressure (MAP, 12 ± 3 mm Hg). When the baroreflex bradycardia and the slope of the cardiac baroreflex curve were examined using a pressor response with brief occlusion of the abdominal aorta, the VTA stimulation blunted both the baroreflex bradycardia and the maximal slope of the baroreflex MAP-HR curve by 63-74% in the same manner as spontaneously-evoked motor activity. In contrast, stimulation of the SN elicited no modulation of cardiac baroreflex. It is likely that stimulation of the mesencephalic VTA suppresses cardiac baroreflex sensitivity and has the similar features of the effects on the cardiac baroreflex function as those during spontaneously-evoked motor activity.